Engineering models for softening and relaxation of Gr. 91 steel in creep-fatigue conditions

Purpose-There are a number of different approaches for calculating creep-fatigue (CF) damage for design, such as the French nuclear code RCC-MRx, the American ASME III NH and the British R5 assessment code. To acquire estimates for the CF damage, that are not overly conservative, both the cyclic material softening/ hardening and the potential changes in relaxation behavior have to be considered. The data presented here and models are an initial glimpse of the ongoing European FP7 project MATISSE effort to model the softening and relaxation behavior of Grade 91 steel under CF loading. The resulting models are used for calculating the relaxed stress at arbitrary location in the material cyclic softening curve. The initial test results show that softening of the material is not always detrimental. The initial model development and the pre-Assessment of the MATISSE data show that the relaxed stress can be robustly predicted with hold time, strain range and the cyclic life fraction as the main input parameters. The paper aims to discuss these issues. Design/methodology/approach-Engineering models have been developed for predicting cyclic softening and relaxation for Gr. 91 steel at 550 and 600°C. Findings-A simple engineering model can adequately predict the low cycle fatigue (LCF) and CF softening rates of Gr. 91 steel. Also a simple relaxation model was successfully defined for predicting relaxed stress of both virgin and cyclically softened material. Research limitations/implications-The data are not yet complete and the models will be updated when the complete set of data in the MATISSE project is available. Practical implications-The models described can be used for predicting P91 material softening in an arbitrary location (n/Nf0) of the LCF and CF cyclic life. Also the relaxed stress in the softened material can be estimated. Originality/value-The models are simple in nature but are able to estimate both material softening and relaxation in arbitrary location of the softening curve. This is the first time the Wilshire methodology has been applied on cyclic relaxation data.